DC 30. +7 bonus. You need to roll 23+ on the dice.
To succeed, the wizard needed to either roll a 20 on the d20 and a 3 or 4 on the d4, or 19 on the d20 and a 4 on the d4.
Chance of getting a 20 is 5%, but even with a 20, half the time the guidance d4 won't be high enough. So 2.5% chance.
Chance...